This invention relates to plug removal apparatus and more particularly to apparatus for removing plugs from plugged heat exchanger tubes.
In tube type heat exchangers, a first fluid flows through the tubes of the heat exchanger while a second fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids. Occasionally, one of the tubes can become defective and leak, resulting in commingling of the two fluids. When this occurs, it is sometimes necessary to plug the tube so that the fluid does not flow through the tube, thereby preventing leakage from the tube.
In nuclear reactor power plants, the tube type heat exchangers are commonly referred to as steam generators. When a defect occurs in the tubes of the nuclear steam generator that allows the coolant in the tubes to mingle with the coolant outside of the tubes, a more significant problem arises. Not only does this situation create an ineffective heat exchanger, but it also creates a radioactive contamination problem. Since the fluid flowing in the tubes of a nuclear steam generator is generally radioactive, it is important that it not be allowed to leak from the tubes and contaminate the fluid surrounding the tubes. Therefore, when a leak occurs in a nuclear steam generator heat exchange tube, the heat exchange tube is plugged so that the coolant is not permitted to flow through the tube. This prevents contaimination of the fluid surrounding the tubes.
There are several kinds of plugs that may be used to plug the heat exchange tubes. One such device used to plug heat exchange tubes in nuclear reactors is an explosive plugging device. With the explosive plugging devices, a metal plug is inserted into the heat exchange tube with an explosive contained within the plug. When the explosive is detonated, the plug is forced into close contact with the inside of the tube, thus blocking flow through the tube. One problem associated with explosive plugging is that should it become necessary to replace the defective tube, a portion thereof, or a defective plug, the explosive plug must be drilled out. Plug removal with conventional drills is time consuming and requires great care for minimization of damage to the tube walls.
A device used more commonly to plug heat exchange tubes is generally referred to as a mechanical plug. In U.S. Pat. No. 4,390,042 to H. D. Kucherer et al., issued June 28, 1983 and entitled "Tube Plug", the disclosure of which is incorporated herein by reference, is described a mechanical plug comprising a cylindrical shell member closed at one end and having a tapered inner bore. An expander member is disposed in the cylindrical member such that when the expander member is drawn along the longitudinal axis of the cylindrical member, the outer surface of the cylindrical member is forced into close contact with the inner surface of the heat exchange tube due to the relative tapers of the inner surface of the cylindrical member and the expander member. In U.S. Pat. No. 4,369,662 to S. L. Rieben et al., issued Jan. 25, 1983 and entitled "Plug Installation Apparatus", the disclosure of which is incorporated herein by reference, is described an apparatus for installing a mechanical plug such as the plug described above by moving the expander member relative to the cylindrical shell member.
In a mechanical tube plug, no explosives are necessary to create the close contact between the plug and the heat exchange tube. Rather, the expansion of the plug is caused by the relative movement of the expander member within the plug. This gives the mechanical plug the characteristic of being able to be removed without being drilled out. U.S. Pat. No. 4,366,617 to W. F. Nanstill et al. issued January 4, 1983 and entitled "Plug Removal Apparatus", the disclosure of which is incorporated herein by reference, describes an apparatus for removing a mechanical plug. This apparatus is adapted to screw into the threaded end of the cylindrical shell member, push the expander relative to the plug to relieve the pressure on the plug, then to extract the plug by pulling.
However, particularly in cases of plugs which have been in service for prolonged periods of operation, the contact force remaining after movement of the expander member may be sufficient to cause scoring of the tube wall upon withdrawal of the plug from the tube. Such tube wall scoring hampers subsequent repair or plug replacement operations. Also, some plugs have become jammed in tubes by improper installationto such an extent that they cannot be pulled out of the tube even after the expander member has been retracted. Also, because the channel head is a relatively small area in which to work, it is required that whatever apparatus or device is used to remove the plugs from the plugged tubes in the tube bundle be capable of being operated in a relatively small space. In addition, because the channel head is radioactive, the device must be easy to manipulate and capable of removing plugs from plugged tubes in a relatively short period of time so as to prevent personnel from prolonged radiation exposure.
U.S. patent application Ser. No. 06/588,422 filed March 12, 1984 in the name of P. J. Hawkins and entitled "Mechanical Plug Drill", the disclosure of which is incorporated herein by reference, describes a lightweight self-centering tube plug drill which solves the problems described above. Therein, a centering and retaining pin is threaded into the threaded portion of the sleeve at the closed end of the plug for pulling the plug out of the tube. However, since not all mechanical plugs are provided with this threaded portion, the device cannot be used to remove many plugs now in service. Therefore, what is needed is an apparatus which is relatively small and easy to manipulate yet is capable of quickly removing any kind of plug from plugged tubes without damaging the walls of the plugged tubes.